1. Field of the Invention
This invention relates to a scanning probe microscope comprising a sharply pointed probe to be placed near the surface of a sample for scanning the surface and for collecting data therefrom.
2. Description of the Related Art
Scanning probe microscopes are known as instruments to observe the atomic state of the surface of samples and include, among others, a scanning tunnel microscope (STM) and an atomic force microscope (AFM).
As described in U.S. Pat. No. 4,343,993 and other documents, an STM is a microscope that utilizes a tunnel current I.sub.T that runs as a tunnel effect between the conductive probe of the microscope placed as close as several nanometers to the conductive surface of a sample at a rate that exponentially depends on the distance S between the probe and the sample and the barrier height .phi. (as expressed by equation I.sub.T =B(V.sub.T)exp(-A.phi..sup.1/2 S); where B(V.sub.T) is a coefficient dependent on the bias voltage, A is a numeral coefficient equal to 10.25.sup.-1 (eV).sup.-1/2, .phi. is the barrier height and S is the distance between the probe and the sample) in such a manner that the tunnel current I.sub.T is kept constant as the surface of the sample is scanned by the probe in order to collect data representing the state and the conductivity of the surface as minute as the level of atomic dimensions.
On the other hand, an AFM is, as described in documents such as U.S. Pat. No. 4,724,318, an instrument designed to detect the deflection (displacement) of its cantilever caused by a minute atomic force (such as a repulsive force attributable to the exclusion principle, a vander Waal's force or a covalent bond force) that can be expressed in terms of Lennard Jones potential and controlling the movement of its probe and hence the positional relationship between the probe and the surface of a sample in such a manner that the force may always be kept constant so that consequently the state of the sample can be observed by a resolution of atomic order.
The tunnel current detected by a STM reflects the state density of local electric charges of the sample as well as local electric potentials and the distance between the probe and the sample. In other words, an image of a sample obtained by a STM normally contains data on the fine configuration or coarseness of the surface, those on the state density of local electric charges and those on local electric potentials of the sample. In order to avoid this problem, there have been developed a technique called scanning tunnel spectroscopy (STS) that can selectively collect data on the electronic state of the surface of a sample by separating them from those on the fine configuration of the surface to produce a three dimensional image (STS image) of the sample and a technique of scanning tunnel potentiometry (hereinafter STP) that collects data on the distribution of electric charges on the surface of a sample to produce a three dimensional image (STP image) of the sample. U.S. patent application Ser. No. 07/585,880, now U.S. Pat. No. 5,185,572, assigned to the same assignee as the present application, discloses an apparatus that can obtain an STS image and an STP image of a sample simultaneously.
The applicant of the present invention has also proposed an AFM/STS system that can determine the surface configuration of a sample by using a conductive cantilever for instrumentation and the principle of AFM and, at the same time, obtain a scanning tunnel microscopic image (STM image) by detecting the tunnel current running between the probe of the microscope and the sample.
With an STM, the configuration of the surface of a sample is determined by so controlling the distance between the sample and the probe of the microscope as to maintain the tunnel current at a constant level. If, however, the surface of the sample has areas that are not or only poorly conductive to electricity, the tunnel current does not will hardly pass through there and the tip of the probe can be pressed against the surface of the sample until the probe and/or the sample are irreversibly deformed. If such is the case, the image obtained by the STM does not correctly reflect the surface of the sample.
While an accident of pushing the surface of a sample with the probe can be avoided in an above described system if the surface of an sample is observed only by an AFM under the control of the AFM servo of the system, the operation of the system in such a manner may be accompanied by problems. When a sample is observed by both the AFM and the STM simultaneously under the control of the AFM servo and the probe comes to a highly conductive area of the surface of the sample, an excessively large electric current can appear due to the exponential dependency of tunnel current on the distance to heat the tip of the probe and thermally change the composition of and/or deform the probe and the sample when a repulsive force involved in the system comes within a range predetermined for the ATM if the distance that triggers a tunnel current exceeds the effective range of the repulsive force defined for the servo by the system.